This invention relates to power boats, such as ski boats having inboard drive systems.
It is known to provide power boats with a boarding platform across the back behind the transom, this boarding platform being provided at a level to be located slightly above water level when the boat is stationary in the water. This enables a skier for example to climb aboard the boat or to enter the water behind the boat with relative ease. In some known boats, particularly some United States designs, such platforms are made of teak and the platforms are secured to the stern as an attachment. Some other boat designs have a relatively narrow platform formed as an integral part of the shape of the stern or transom area.
One common problem associated with such boarding platforms across the back of power boats is that upon rapid deceleration a xe2x80x9cstern wavexe2x80x9d can flow over the platform and can rise or splash up over the stern and into the boat. To explain this more fully, when a power boat travelling at speed is quickly decelerated and the boat rapidly settles back down into the water from its elevated planing position, the xe2x80x9cstern wavexe2x80x9d that forms immediately behind the trough in the water surface behind the stern of the boat can catch up with and overtake the boat thus overflowing the boarding platform and splashing up or rising over the transom.
It is an object of the present invention to provide a power boat having features which can alleviate the abovementioned problem or disadvantage associated with the stern wave flowing over a boarding platform.
According to the present invention there is provided a power boat including a bow and a stern, the boat having a hull with a longitudinal dimension extending from the bow of the boat to the stern of the boat, the boat also having a platform extending across the stern for enabling boarding of the boat, the platform being located substantially at or slightly above the water surface when the boat is stationary in the water, the hull having a planing zone located at a stern region of the hull in the vicinity of the stern of the boat, the planing zone being shaped so that a longitudinal line extending along the surface of the hull in the direction of the longitudinal dimension from the bow to the stern upon reaching the planing zone turns upwardly towards the water surface relative to the longitudinal line forwardly of and before reaching the planing zone, the planing zone providing a zone of the hull where, when the boat is travelling through the water at speed, drag of the water travelling relatively along the planing zone assists stable travel of the boat through the water.
It should be noted that in referring to the longitudinal line extending along the hull as turning xe2x80x9cupwardlyxe2x80x9d, it is not intended to limit the direction of the longitudinal line to being inclined upwardly relative to a horizontal line and towards the water surface. For example, if the longitudinal line extending along the hull surface forward of the planing zone is inclined downwardly in a direction from the bow to the stern (considering tangents to the hull at points along the longitudinal line), the line could turn back towards a horizontal direction upon reaching the planing zone and this would still qualify as xe2x80x9cturning upwardly towards the water surface relative to the longitudinal line forwardly of and before reaching the planing zonexe2x80x9d. This interpretation of the expression xe2x80x9cturns upwardlyxe2x80x9d applies throughout this specification, including the claims.
Preferably the planing zone is located below the boarding platform. In this embodiment, preferably the boat includes a transom extending across the stern of the boat and which rises upwardly from a forward end of the boarding platform, the hull including a transition region where the longitudinal line extending along the hull surface turns relatively upwardly and demarcates the start of the planing zone, the transition region being located generally directly below the transom.
The power boat includes drive means preferably including a drive motor and drive components beneath the boat for imparting thrust to the boat. In the preferred embodiment, the drive motor is located inboard and may for example comprise an engine located in a mid position along the longitudinal length of the hull. Preferably the drive components beneath the boat are located beneath the planing zone of the hull. In the preferred embodiment this will entail locating the drive components beneath the boarding platform. Alternatively the drive components may be located generally below the transom immediately forward of the boarding platform.
The drive components may include a propeller mounted to a drive shaft extending from the drive motor, the drive shaft and propeller being located and supported in their desired position beneath the planing zone by a skeg projecting downwardly from the hull, the skeg having a bearing at its bottom extremity through which the drive shaft extends, the propeller being located immediately astern of the skeg. Because in the preferred embodiment the propeller is located at a position below or behind the transom and beneath the boarding platform, the drive shaft will be longer than an otherwise similarly dimensioned and configured power boat and the drive shaft will extend at a shallower angle to the horizontal because it is effectively extending from the drive motor to a rearwardly displaced propeller. For example, the drive shaft may be at an angle of about 12xc2x0 to the horizontal calculated when the boat is stationary. By providing the relatively shallow angle of the drive shaft, a greater relative component of thrust produced is in a forwards direction, i.e. there is less lifting thrust and this may enable greater travelling speed to be achieved compared to an otherwise similarly configured and powered power boat. Also, the effective displacement of the propeller further astern and the provision of the drive motor generally midway along the length, will mean that the centre of mass or point of balance of the boat is relatively further forward. This might normally be expected to cause the boat to drop or pitch over its point of balance at speed but it has been found that by inclining the planing zone commencing at the skeg relatively upwardly towards the water surface, the planing angle of the boat is nevertheless maintained at high speed. Furthermore, when the boat decelerates and drops down from its planing angle, the boat adopts a very flat position and only a very weak stern wave is generated. In a prototype of the boat according to the invention, upon the boat dropping down rapidly from high speed planing, there was insufficient energy in the stern wave to mount the boarding platform.
To further assist the travelling stability and drive efficiency, strakes are preferably provided on the hull to provide lift to the hull at positions forwardly of the planing zone by deflecting water laterally as the hull travels through the water. In the preferred embodiment, the hull is generally V-shaped in transverse section having a keel at its lowest point, the strakes being provided at each side of the hull displaced laterally from the keel and extending generally longitudinally but terminating before reaching the planing zone whereby there is increased drag astern of the strakes and at the stern region of the hull. As the boat hull travels at speed through the water, the water relatively flowing along the hull from the elevated bow encounters the strakes and is deflected laterally thus reducing drag on the hull and providing lift for the bow end of the hull.